Swine flu has become the world's first official pandemic in 41 years after the World Health Organisation raised the alert level on the H1N1 virus from phase 5 to 6, the highest on the scale.

The decision was made after a meeting of experts in Geneva today and comes as infections have climbed in Europe, the US, Australia, South America and elsewhere. The WHO chief, Dr Margaret Chan, yesterday examined data from eight countries with large numbers of swine flu cases.

"The world is moving into the early days of its first influenza pandemic in the 21st century," Chan told reporters tonight, advising that countries should prepare for a second wave of cases. "The [swine flu] virus is now unstoppable."

Raising the alert level reflects that the virus is more widespread, but not necessarily more dangerous – although there are fears that infections could overwhelm hospitals and health authorities, especially in poorer countries.

Another 25 cases of swine flu have been confirmed in England and 26 more in Scotland, taking the UK total to 848.

Speaking to health service managers in Liverpool at a conference of NHS trusts, the UK's new health secretary, Andy Burnham, said: "It's not a cause for alarm. It doesn't change our plans."

Swine flu originated in Mexico in April and has spread to 74 countries, infecting more than 27,700 people and killing 140.

Most cases have been mild, although employers are being warned to prepare for absences through illness.

The WHO said pharmaceutical companies should start making swine flu vaccine. One manufacturer, GlaxoSmithKline PLC, said it could start large-scale production by July but large quantities would not be available for several months.

Despite the WHO's hopes, the announcement of a pandemic will almost certainly spark panic in some countries. Fear has already gripped Argentina, where thousands of people worried about swine flu flooded into hospitals this week, bringing to near-collapse the emergency health services in the capital, Buenos Aires.

"People might imagine a virus is now going to rush in and kill everyone," said John Oxford, a professor of virology at St Bart's and Royal London hospital. "That's not going to happen."

There are 337 cases in Scotland and health officials in Edinburgh said 30% of them were in young adults aged 15-24.

The Scottish health secretary, Nicola Sturgeon, said public health tactics had changed after it emerged the virus was spreading uncontrolled.

Sturgeon told the Scottish parliament this morning that hospitalisation rates were similar to those in the US. She indicated that attempts to contain the virus had failed and the strategy would be to limit its spread. Doctors in the most-affected areas – Glasgow, Dunoon and Paisley – would be allowed to make a swine flu diagnosis in their surgeries rather than wait for specialist or laboratory tests.

England's chief medical officer, Sir Liam Donaldson, said health officials were expecting to see more and more cases this autumn and winter with the return of the traditional flu season, and the virus might yet change and become more severe.

There were "very strong" plans in place to deal with the flu and so far the government's approach, including closing schools where necessary, appeared to have worked well, he said.

At the NHS Confederation conference, senior health service managers held an early-morning session to refine their emergency plans. Contingency arrangements are being drawn up to allow for hospitals and health centres to continue operating when as many as 40% of staff are off sick.

Steve Barnett, the chief executive of the NHS Confederation, which represents most health and ambulance trusts, said: "The confirmation of a level 6 global pandemic reinforces the need for the NHS to ensure all the flu plans already in place at local level are as comprehensive as possible and thoroughly tested.

"We need to avoid complacency in dealing with a virus that is an unknown and seems to be spreading quickly."

The last such pandemic was the Hong Kong flu of 1968 that killed about a million people. Ordinary flu kills about 250,000 to 500,000 people each year.